This invention relates to a method of manufacturing a superconductive coil possessing high critical electric current density by explosive compaction.
It is generally known that chemical compounds having a perovskite structure formed of rare earth elements (hereinafter abbreviated as "R") including yttrium, alkali earth metals, copper, and oxygen (hereinafter referred to as "R-based oxides) exhibit superconductivity at a temperature of approximately 77.degree. K., to which they can be cooled by means of liquid nitrogen.
Conventionally, a superconductive coil has been manufactured from a powder of an R-based oxide in the following manner: An R.sub.2 0.sub.3 powder, an alkali earth metal carbonate powder, and a copper oxide (CuO) powder are used as starting materials, each having a mean particle size of 10 microns or less. The starting material powders are used in predetermined proportions, and mixed, and the resulting mixture is sintered in an air or oxygen atmosphere at a temperature within a range from 850.degree. to 950.degree. C. into a sintered body of the R-based oxide having a perovskite structure. The R-based oxide sintered body is crushed into a powder having a mean particle size of 10 microns or less to obtain an R-based oxide powder. The R-based oxide powder is charged into a tube formed of silver, and then the opposite ends of the tube are closed. The tube is then subjected to drawing, e.g., by means of swaging, rolling with a grooved roll, or die drawing, into a composite wire of the R-based oxide and silver with an outer diameter of 5 mm or less. The composite wire is then formed into a coil, and then heat-treated in an air or oxgen atmosphere at a temperature within a range from 900 to 950.degree. C. to obtain an R-based oxide superconductive coil.
Further, it has recently been found that Bi-Ca-Sr-Cu-O oxides (hereinafter referred to as "Bi-based oxides"), and Tl-Ca-Ba-Cu-O oxides (hereinafter referred to as "Tl-based oxides") exhibit superconductivity at a temperature of 77.degree. K. or higher, to which they can be cooled by means of liquid nitrogen.
To obtain a Bi-based oxide for a Bi-based oxide superconducting coil, first a Bi.sub.2 0.sub.3 powder, a CaCO.sub.3 powder, an SrCO.sub.3 powder, and a CuO powder are used as starting material powders. The starting material powders are used in predetermined proportions, and mixed, and the resulting mixture is sintered in an air or oxygen atmosphere at a temperature within a range of 700.degree. to 800.degree. C. for 4 to 12 hours. On the other hand, to obtain a Tl-based oxide, a Tl.sub.2 0.sub.3 powder, a CaCO.sub.3 powder, a BaCO.sub.3 powder, and a CuO powder are used as starting material powders. The starting material powders are used in predetermined proportions, and mixed, and the resulting mixture is sintered in an air or oxygen atmosphere at a temperature within a range of 600.degree. to 700.degree. C. for 4 to 12 hours.
The Bi-based oxide or Tl-based oxide thus obtained is crushed into a Bi-based oxide powder or Tl-based oxide powder having a mean particle size of 5 microns or less. The obtained Bi-based oxide powder or Tl-based oxide powder is charged into a silver tube, and then the opposite ends of the tube are closed. The tube is then subjected to drawing into a composite wire of the Bi-based oxide and silver or a composite wire of the Tl-based oxide and silver with an outer diameter of 5 mm or less. The obtained composite wire is formed into a coil, and heat-treated in an air or oxgen atmosphere to obtain an Bi-based oxide superconductive coil or a Tl-based oxide superconductive coil, wherein the heat-treatment temperature ranges from 830.degree. to 870.degree. C. for the Bi-based superconductive coil or from 880.degree. to 920.degree. C. for the Tl-based superconductive coil.
The conventional superconductive coils have insufficient critical electric current densities for practical use in various electrical apparatus. That is, the R-based oxide superconductive coil has a critical electric current density of about 700A/cm.sup.2 at the most, the Bi-based oxide superconductive coil about 100A/cm.sup.2 at the most, and the Tl-system oxide superconductive coil about 180A/cm.sup.2 t at the most. Under the situation, there is a demand for development of a superconductive coil which has higher critical electric current density.
To meet the demand, it has been proposed to subject an R-based oxide superconductive coil, a Bi-based oxide superconductive coil, or a Tl-based oxide superconductive coil to explosive compaction so as to increase the density of the oxide powder within the coil and hence the critical electric current density of the superconductive coil. However, according to the proposal, since the coil is directly subjected to explosive compaction, it can be heavily deformed or even broken during explosion. Thus, the conventional superconductive coils do not meet the requirements for practical use in the above-mentioned apparatus.